This invention relates to generating coherent electromagnetic radiation from charged particle beams and, more particularly, to generating microwaves from relativistic electron beams which form an oscillating virtual cathode. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Relativistic electron beam devices can form oscillating virtual cathodes, a nonlinear state arising when the electron beam current exceeds the space-charge limiting current as defined by the beam energy and system geometry. Oscillation of the virtual cathode generates microwaves in a waveguide. U.S. Pat. No. 4,345,220 to Sullivan teaches various theoretical aspects concerning the formation of a virtual cathode and is incorporated herein by reference. Experiments in conventional configurations indicate that this class of microwave generator produces microwaves at multiple frequencies and modes, operates at relatively low efficiencies, and the output usually shows bursts or erratic variation with time.
The complex behavior of virtual cathode devices may be attributed to microwaves generated by two distinct processes arising from the virtual cathode. As noted above, the oscillating virtual cathode itself generates microwaves since the space-charge formed at the virtual cathode moves with time. The space-charge, however, also generates repulsive forces for electrons at the virtual cathode, reflecting some electrons back to the real cathode. Electrons can become trapped in this manner, reflexing between the real and the virtual cathodes. The reflexing electrons also generate microwaves which can interfere destructively with microwaves arising from the oscillating virtual cathode.
It has been suggested that microwave generation from a virtual cathode generator might be coherent and efficient if electron reflexing into the diode region (i.e., the region between the cathode and anode) is prevented. See Donald J. Sullivan, "High Power Microwave Generation From A Virtual Cathode Oscillator (Vircator)," IEEE Transactions on Nuclear Science, Vol. NS-30 No. 4, 3426-3428 (August 1983), incorporated herein by reference. Sullivan, however, proposes a foilless diode with a strong axial magnetic field where an ungrounded collimator might be used to help prevent reflexing of electrons back to the cathode. No experimental or simulated parameters are provided, however, to indicate operability of such a device and, indeed, foilless diodes are known to generate an unstable virtual cathode with attendant random microwave generation and inefficiency.
An anode foil with narrow slits was suggested by Thomas J. T. Kwan, "High-Power Coherent Microwave Generation From Oscillating Virtual Cathodes," Phys. Fluids, 27 (1), 228-232 (January 1984), for absorbing a majority of the reflected electrons. It was suggested that suppression of the reflexing electrons could lead to more efficient microwave generation. There are no simulation parameters or results associated with this suggested configuration. Accordingly, it would be desirable to provide an efficient and coherent generator of microwaves using oscillating virtual cathodes. Such a device is obtained in accordance with the present invention and microwave production is confined to an oscillating virtual cathode for efficient and coherent generation.
Thus, it is an object of the present invention to develop an efficient microwave generator using an oscillating virtual cathode.
Yet another object of the present invention is to generate microwaves from a virtual cathode source with a narrow bandwidth spectrum and with most of the energy radiated in a few modes.
One other object of the present invention is to develop an operable microwave generator eliminating reflexing electrons as a microwave source.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.